clean up stale aic_btusb references on bluetooth branch (#53)

This branch already switched to standard btusb (commit 86aa95f), but
several pieces still pointed back at the removed aic_btusb module:

- drivers/aic8800/aic_btusb/ source tree (no longer in build)
- modprobe/aic8800-bt.conf installed to /etc/modprobe.d/, which
  softdep'd btusb to a nonexistent aic_btusb and aliased AIC BT
  interfaces to it. This caused btusb to load without firmware
  having been uploaded, producing HCI_Reset timeout -110 on hci0.
- install.sh still copied that conf into /etc/modprobe.d/
- diagnose_bt.sh still told users to modprobe aic_btusb

diagnose_bt.sh rewritten to flag the residual /etc/modprobe.d/aic8800-bt.conf
on already-installed systems and to advise the correct flow
(aic_load_fw uploads BT firmware, btusb takes over).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

install.sh

@@ -289,18 +289,6 @@ install_firmware() {
         print_warning "Udev rules file not found: $rules_source"
     fi
     
-    # Install modprobe configuration for Bluetooth
-    local modprobe_source="${SCRIPT_DIR}/modprobe/aic8800-bt.conf"
-    local modprobe_dest="/etc/modprobe.d/aic8800-bt.conf"
-    
-    if [ -f "$modprobe_source" ]; then
-        print_info "Installing modprobe configuration to $modprobe_dest..."
-        cp "$modprobe_source" "$modprobe_dest" >> "$LOG_FILE" 2>&1
-        print_success "Modprobe configuration installed successfully."
-    else
-        print_warning "Modprobe configuration file not found: $modprobe_source"
-    fi
-    
     # Install usb_modeswitch configuration
     local modeswitch_source="${SCRIPT_DIR}/usb_modeswitch/1111_1111"
     local modeswitch_dest="/etc/usb_modeswitch.d/1111:1111"